Creasing of cardboard to obtain a structured pattern of creasing lines is typically performed to facilitate the subsequent folding of the cardboard. As an example, creased cardboard, being part of a packing material and having a structured pattern of creasing lines, may be folded into a package. Typically, blanks to form a package are creased once still being a part of wide web. The wide web may comprise at least two lanes of blanks. Thus, identical structured pattern of creasing lines may be present in lanes adjacent to each other.
Packages for liquid food products have to be essentially liquid proof. For many years packaging materials comprising an innermost thermoplastic layer, to be in contact with the liquid food, a cardboard web, a barrier material, arranged between the innermost thermoplastic layer and the cardboard web, and an outermost thermoplastic polymer layer have been used for this purpose with an excellent result. When producing packages of increased volumes, for instance such containing 1.5 liters or more, the stress on the material however increases and thus the thickness of the cardboard has to be increased.
As a result of the increased thickness there is a tendency of the cardboard web while influenced by male and female dies, as well as different types of rollers in the creasing process, which is positively progressing, to create what is called wild-creases. A wild-crease is an unwanted self-generated crease-like deformation or a defibration of a cardboard material between two close neighboring creases intentionally made. The degree of wild-creasing seems to be the affected by the compression stresses employed to create the intentional creases, the mutual distance between the separated neighboring open ended creases, the depth of each crease and of course the thickness of the creased material. This gives rise to shear stresses in the pulp layers of the cardboard material which may cause delamination or defibration thereof which in turn may produce the wild-creases. The wild-creases occur mainly between two coherent lanes in the modified offset printing process that the process of creasing as a matter of fact is. Typically, wild-creasing occur between the open ends of the adjacent creasing lines (cf. FIG. 1), being perpendicular to the direction of the motion of the cardboard web in the creasing process. The appearance of the wild-creases are thus as an uncontrolled wrinkling of the cardboard in an area between two from one another independent co-linear male dies and more specifically between two separated crease lines.
If wild-creases occur on a package blank in an area where a longitudinal seal is to be made then there is a risk that the package thus produced will be prone to start leaking. The creation of wild-creases here and elsewhere may as another consequence lead to loss of product integrity. These phenomena are of course not acceptable, neither from a producer point of view, nor from the same of a consumer.
As disclosed in WO 2006/112767, the problem of wild creasing may be overcome by providing areas of each the female dies, corresponding to areas where the male dies show interruptions, especially those that run transversally to the running direction of the machine, with a cantilever arrangement. The cantilever arrangement has been shown to be effective in preventing or at least reduce wild creasing. While the cantilever arrangement is effective in preventing wild creasing, it is an expensive and complex technique. Further, it typically requires modification of cardboard creasing machine.
It would be advantageous if wild creasing of cardboard in packaging materials may be prevented, or at least reduced, in an alternative manner, such that the need to provide the female dies with cantilever arrangements may be dispensed with.